The invention relates to bearings, and more especially to tilting pad journal bearings.
It is well known in the art that in some instances rotors run unstably in simple journal bearings of circular bore, especially when the loading of the bearings is only a relatively small fraction of their load capacity. In many bearing applications, more elaborate forms of journal bearing are used to promote stability in operation. One such form of bearing for a journal is a tilting pad journal bearing, which generally comprises three or more pads located around the journal within a rigid bearing shell. The inwardly-facing surfaces of the pads conform approximately to the outer surface of the journal, and the pads are so arranged that they can pivot relative to the rigid bearing shell and align themselves with the journal, in operation. The design of the bearing may be such that, as the journal rotates, lubricating fluid present in the shell is drawn between the journal and the pads. Above a threshold rate of rotation, a continuous film of lubricant is formed which prevents solid to solid contact between the pads and the journal. Such an arrangement is referred to in the art as being xe2x80x9cself-generatingxe2x80x9d.
The invention provides a tilting pad journal bearing comprising a bearing shell having a substantially cylindrical inner surface and at least three bearing pads extending around the inner surface of the bearing shell in a substantially circumferential direction, each of the pads being arranged for pivotal movement relative to the bearing shell by means of a substantially spherically-extending surface on the pad engaging a mating substantially spherically-extending surface associated with the bearing shell, the bearing being a self-generating bearing in which gas films between the pads and a journal are self-generated in use when the journal is rotated above a threshold rate, wherein at least one of the said substantially spherically-extending surfaces associated with the shell is provided on a member so mounted in or on the bearing shell as to be capable of radial movement relative to the shell and wherein the bearing further comprises spring means for urging the said member radially inwardly and gas supply means for supplying pressurized gas to the interface between the journal and the pad associated with the said member to create in use a separating film aerostatically at said interface to urge the said member radially outwardly, said spring means and said gas supply means being arranged in use to separate the pad associated with the said member from the journal when the journal is stationary and at least up to said threshold rate of journal rotation.
The expression xe2x80x9cspherically-extending surfacexe2x80x9d is used in this specification to mean a surface that is at least a part of the outer surface of a sphere or at least a part of the inner surface of a hollow sphere.
The or each of the substantially spherically-extending pivot surfaces on the pads may be concave surfaces, the mating surfaces associated with the bearing shell being convex surfaces. Alternatively, the or each of the pads may be provided with convex pivot surfaces, the mating surfaces associated with the bearing shell being concave.
With the arrangement of the invention, means are provided for supplying high pressure gas to the interface between the bearing pads and a journal supported by the bearing, in use. Such a supply urges the bearing pads radially outwardly.
With previously proposed arrangements of tilting pad journal bearings that are self-generating, a film of lubricating fluid between a journal and its bearing formed by the rotation of the journal is able to produce separating forces sufficient to hold the pads firmly in position above a certain rotational speed of the journal. When operation is initiated and at low rotational speeds, however, and especially in arrangements in which the lubricating fluid is a gas, the or each pad not supporting the weight of the journal is not held steady, and may be prone to flutter.
With the arrangement of the invention, one or more radially-movable members mounted in or on the bearing shell and urged radially inwards can be provided to engage the or each pad that might otherwise be prone to flutter at low rotational speeds, so that the or each of the said pads is, in turn, urged radially inwards. This radially inward urging acts in opposition to the radially outward urging of the high pressure gas supplied to the interface between the bearing pads and the journal. These oppositely directed urgings act to steady said pad or pads that might otherwise be prone to flutter. In addition, the radially outward urging is greater than the radially inward so that, before rotation is initiated and at least up to the threshold speed at which xe2x80x9cself-generationxe2x80x9d occurs, a separating film of gas is created aerostatically to separate said pad or pads from the journal to provide protection of the co-operating surfaces of said pad or pads and the journal at low rates of rotation and before adequate self-generation arises.
The bearing of the invention can operate, even initially, with substantially constant loading of all of its pads. This provides improved control of the journal, and flutter of one or more of the pads can be inhibited.
In operation, rotors are subject to synchronous vibration caused by unbalance of the rotor, such vibration being referred to as xe2x80x9cforced vibrationxe2x80x9d. In some instances, a rotor is also subject to non-synchronous vibration at a frequency related to a natural frequency of the rotor, and that vibration can be excited by an input of energy at a bearing or bearings. That kind of vibration is referred to as xe2x80x9cself-excited vibrationxe2x80x9d. The arrangement of the invention can help to moderate the tendency towards self-excited vibration by maintaining substantially uniform loading of the pads of the bearing at all stages of operation.
Advantageously, the number of pads that is provided with radially-movable members and means for urging the members radially inwards is at least that number required for each pad to be held positively when the journal is stationary. Preferably, at least the or each pad not arranged to bear the weight of the journal when the journal is stationary is provided with such means. By way of example, when a bearing has three equally-spaced pads of equal length it is sufficient for one pad to be provided with a radially-movable member if the other two pads are positively located by the weight of the journal, but in the case where there are four equally spaced pads of equal length, then two or more of the pads should generally each be provided with a radially-movable member depending on whether the axis of the rotor is horizontal or vertical, and, if the axis is horizontal, on whether the weight of the rotor is shared equally by two pads, or is taken principally by one of them.
The bearing of the present invention can be so arranged that, in operation, as the speed of rotation of the journal is increased, separating forces exerted by self-generated fluid films between the journal and the pads rise sufficiently to overcome substantially constant pad loading exerted by the or each radially-movable member, and the or each radially-movable member moves further radially outwards. This will, of course, depend upon several factors including the force urging the members inwards, and the weight of the pads themselves. Advantageously, there is provided means for limiting radial movement of the or each of the said members relative to the bearing shell in an outward direction beyond a limiting position. With such an arrangement, when the or each of the said members reaches its limiting position, further retraction is prevented and the bearing can then act as a bearing in which all the pads have pivots that are at fixed positions relative to the bearing shell.
The arrangement of the invention is applicable to bearings that operate with liquid or gas as their lubricant, but is especially advantageous in a bearing that is lubricated with gas, for example, air, in operation.
If the speed of rotation of a rotor coincides with a natural frequency of vibration of the rotor in its bearings, it is said that the rotor is running at a xe2x80x9ccritical speedxe2x80x9d. It is desirable to avoid that, but with a variable speed machine, it is not always possible. In designing a rotor system including its bearings, in order to avoid the normal running speed or speeds of the rotor being critical speeds, it is first necessary to determine the critical speeds, which depend upon the characteristics of the bearings. As the speed of rotation of a rotor is increased through its threshold speed at which self-generated films are created to its desired operating speed, it may have to pass through one or more critical speeds. By maintaining the supply of high pressure gas to the interface between the bearing pads and the journal even after reaching the threshold speed and as the rotor passes through its critical speed or speeds, as is possible with the arrangement of the invention, increased stability can be given to the rotor during its acceleration to its full operating speed.
When the bearing is provided with means for supplying high pressure gas to the interface between the bearing pads and the journal as referred to above, the arrangement is preferably such that the high pressure gas is supplied to the said interface by passing substantially diametrically through the said mating substantially spherically-extending surfaces of the pivots. The said means for supplying high pressure gas preferably comprises a passageway extending through the or each radially-movable member and a passageway extending through each bearing pad. A flow resistance, which may be an orifice of restricted cross-sectional area, may be provided in the passageways either upstream or downstream of the substantially spherically-extending surfaces of the pivots. The gas pressure downstream of such a flow resistance will rise when the gap between the pad and its journal diminishes, and the film of gas between the journal and the pad then exerts a restoring force on the pad.
Advantageously, the or each of the said radially-movable members comprises a piston which is arranged as a sliding fit in a radially-extending bore in the bearing shell. Preferably, for ease of manufacture, each of the pads of the bearing is associated with a piston mounted in a radially-extending bore in the bearing shell. In such an arrangement, each of the substantially spherically-extending pivot surfaces associated with the bearing shell is advantageously formed on an inwardly facing surface of one of the pistons. Advantageously, the means for urging each of the said radially-movable members inwards comprises a compression spring.
The invention also provides a rotor system comprising a rotor having a journal and a tilting pad journal bearing in accordance with the invention.
The invention further provides a compressor including a rotor in two or more bearings, at least one of which is a tilting pad journal bearing in accordance with the invention.